The present invention relates to a method for the preparation of a planographic printing plate or, more particularly, to a method for the preparation of a printing plate suitable for dry planographic printing without the use of dampening water in a simple and convenient plate-making process free from the problems in the prior art.
Hitherto, there have been proposed methods for the preparation of printing plate for dry planographic printing utilizing various principles. Nevertheless, none of them is quite satisfactory from the practical standpoint due to their respective drawbacks such as inferior performance as a printing plate or due to the excessively complicated plate-making process.
For example, there are known processes utilizing a diazo type photosensitive composition. Japanese Patent Publication No. 44-23042 teaches a process in which a master plate is prepared by providing two successive layers of a diazo photosensitive layer and a layer of a dimethylpolysiloxane gum on a base plate such as an aluminum plate and the matter plate is exposed to light through a positive transparency overlaid in contact therewith to insolubilize the diazo photosensitive composition in the irradiated areas followed by removal of the diazo composition in the unirradiated areas by dissolving away in a development process and peeling of the layer of the dimethylpolysiloxane gum in the unirradiated areas off the plate.
Further, Japanese Patent Publication No. 46-16044 discloses a process in which a master plate is prepared by providing three successive layers of a diazo photosensitive layer, a layer of an adhesive agent and a layer of a silicone gum on a base plate such as an aluminum plate and the master plate is exposed to light through a negative transparency overlaid in contact therewith followed by development treatment utilizing the photode-composition of the photosensitive diazo composition in the irradiated areas and peeling of the layer of the silicone gum in the irradiated areas off the plate to give a planographic printing plate.
Both of the above described diazo processes have several serious problems. Firstly, the pattern-wise exposure to light for reproducing the images on the positive or negative transparency cannot have a satisfactory accuracy or fidelity because of the presence of the intervening layer of the non-photosensitive silicone gum between the photosensitive diazo layer and the transparency bearing the images. Secondly, the image patterns formed by the layer of the silicone gum after peeling cannot be so sharp and clear-cut in the edge lines because peeling of the layer of the silicone gum is carried out by utilizing the changes of solubility of the photosensitive layer in a solvent. In addition, these processes are disadvantageous due to the complicated steps in plate-making including preparation of the master plate having two or three successive layers on a base plate, exposure to light and development.
Some of the inventors have previously proposed a process in which the image patterns on a printing plate are formed by dissolving away the uncured photosensitive silicone resin on the unirradiated areas by the exposure to light and development on a master plate prepared using a photosensitive silicone resin (see Japanese Patent Disclosure No. 48-33910).
This process is, however, also defective because the problem of the complicated plate-making steps is not solved including the removal of the silicone layer on the image areas by a wet process of development following the exposure to light to give a desired planographic printing plate.
In contrast to the above described known processes for plate-making in which a step of treatment in wet is included, there are known several plate-making processes carried out in dry and most of such dry processes utilize a principle according to which the image patterns are formed with a tonor powder on the surface of a cured silicone layer provided on a base plate by utilizing the techniques of electrostatic photography.
One of the defects in the printing plates prepared by such a tonor process is the poor durability of the printing plate in the printing process due to the low adhesive bonding strength between the tonor and the surface of the cured silicone layer along with the less satisfactory printability of the plates.
For example, Japanese Patent Disclosure No. 48-19305 discloses a plate-making process for dry planography according to which a master plate is prepared by providing a substrate with successive layers of a substance for electrostatic latent image formation and a curable silicone resin with subsequent curing of the latter and tonor images are formed theron electrophotographically and bonded to the surface of the plate by thermal fusion to give a desired printing plate for dry planographic printing. A defect of the thus prepared printing plates is the poor durability of the plate in printing due to the incomplete adhesive bonding of the tonor to the surface of the plate since the silicon resin forming the surface layer has been cured prior to image formation.
Accordingly, various means have been proposed to improve the adhesive bonding of the tonor material and the surface of the silicone layer though with their respective drawbacks. For example, Japanese Patent Disclosure No. 49-21204 discloses a method in which a planographic printing plate is prepared by transferring and bonding by thermal fusion the tonor images formed on the electrophotographically photosensitive layer on to a layer formed of a mixture of a silicone gum and a resin. The planographic printing plate obtained in this manner is defective in its rather poor ink repellency since the surface layer is formed by admixing a resinous component with an object to improve the adhesive bonding between the tonor and the surface of the silicone layer.
Further, Japanese Patent Disclosure Nos. 50-71405, 50-78403 and others disclose a method for the preparation of a planographic printing plate by utilizing an uncured or semi-cured silicone on the substrate, which is subsequently cured after electrophotographic transfer and thermal fixing of tonor images. The planographic printing plates of this type are disadvantageous due to their low storability with the silicone layer as uncured or semi-cured because the thermosetting silicone resin used in the silicone layer is gradually crosslinked even without heating leading to the loss of the adhesivity to the tonor material in the lapse of time.
Therefore, a somewhat troublesome means is undertaken in some of the planographic printing plates by the above principle that curing of the silicone layer is effected by applying a solution of a thermal curing catalyst only after the transfer of the tonor images by the electrophotographic technique.
Furthermore, Japanese Patent Disclosure No. 51-16105 teaches a method for the preparation of a master plate for printing and a printing plate in which a coating layer is formed on a substrate of a silicone gum composition containing two kinds of catalysts and one of the catalysts is activated by exposure to light while the other is activated thermally to cure the silicone gum. This method is, however, defective because, in addition to the troublesome formulation of the photo-activated catalyst such as a reactive azide compound and the thermally activated catalyst such as an organic peroxide, a silicone gum composition containing suh a combination of the catalysts cannot be stored with stability for a long period of time and the heating step is also indispensable when sufficient cure of the silicone gum is desired.
A further improved method is described in Japanese Patent Disclosure 51-134204 for the preparation of a plate material for printing plates suitable for peeling development. The plate material utilizes a layer of a photocurable silicone gum composed of a silicone gum and a photopolymerizing ingredient or a crosslinkable monomer or oligomer sandwiched between two substrates each having a first or a second treated layer with specific properties facing the layer of the photocurable silicone gum and the image-wise exposure of the plate material to light produces a difference between the adhesive bonding strengths of the layer of the silicone gum to the first and the second treated layers. The photocurable silicone gum composition sandwiched between the substrates has a property that the adhesive bonding strength thereof to the first treated layer is decreased by exposure to light to give increased peelability in the irradiated areas. When a printing plate of negative images relative to the original is desired in this method, a second exposure to light of the uncured silicone gum layer is necessary which unavoidably results in a marked decrease in the bonding strength between the cured silicone gum layer and the first treated layer so that the usefulness of this method is limited by a relatively low durability of the printing plate in the printing run. A similar method described in Xerox Disclosure Journal, volume 1, No. 2, 1976, page 15 by R.G. Crystal is also not free from the problem of the decreased adhesive bonding strength caused by the second exposure to light between the cured silicone gum layer and the base plate.
Some of the inventors conducted extensive investigations in view of the above described problems to provide a simple and convenient method for the preparation of a printing plate suitable for dry planographic printing and have proposed a method according to which a printing plate for dry offset printing is obtained.
According to the method described in U.S. Pat. Nos. 4,169,731 and No. 4,184,873, a planographic printing plate is prepared by the steps of
(a) coating one surface of substrate which is transparent to ultraviolet light with an uncured photocurable silicone,
(b) bringing the thus coated surface into direct contact with a surface of a base plate to form a laminate,
(c) providing on the other surface of the substrate an image pattern made of a material which is opaque to ultraviolet light,
(d) irradiating the laminate with ultraviolet light from above the image pattern and
(e) separating the substrate from the base plate to allow a portion of the photocured silicone coating to transfer to the surface of the base plate, and a portion of the uncured silicone coating to stay in situ on the surface of the substrate. The image pattern made of a material which is opaque to ultraviolet light used in the step (c) above is positive relative to the printed images. This method is not free from a problem in the poor bonding on to the base plate by dry laminating since the cured silicone layer finally left on the base plate is formed by dry lamination in an uncured state followed by photocuring.
On the other hand, there has been a demand for image patterns formed on a base plate with a similarly cured silicone material by use of a negative transparency relative to the printed images in order to obtain versatility in the originals for printing which may be positive or negative. The inventors have conducted extensive investigations to comply with this demand and proposed a method (see Japanese Patent Disclosure No. 54-121804). The method comprises the steps of (a) bringing a UV-transparent sheet into direct contact with a layer of a photocurable silicone provided on a base plate, irradiating the photocurable silicone through an image pattern opaque to ultraviolet light provided on the sheet, (c) peeling the sheet together with the silicone photocured image-wise to leave uncured silicone on the base plate and (d) subjecting the uncured silicone on the base plate to photocuring by irradiating with ultraviolet light so as that the resultant plate has image areas where the surface of the base plate is exposed bare and non-image areas where the base plate is covered with the photocured silicone.
The planographic printing plate prepared as described above suffers from a problem of insufficient adhesive bonding of the photocured silicone to the base plate because the adhesive performance of the silicone is so designed that the silicone transfers from the surface of the base plate to the bottom surface of the peeling sheet when the silicone is photocured by pattern-wise exposure to light.